wallet: add command and RPC to sign/verify data

Signing is done using the spend key, since the view key may
be shared. This could be extended later, to let the user choose
which key (even a per tx key).
simplewallet's sign/verify API uses a file. The RPC uses a
string (simplewallet can't easily do strings since commands
receive a tokenized set of arguments).

src/wallet/wallet_rpc_server_error_codes.h

@@ -39,3 +39,4 @@
 #define WALLET_RPC_ERROR_CODE_TRANSFER_TYPE           -6
 #define WALLET_RPC_ERROR_CODE_DENIED                  -7
 #define WALLET_RPC_ERROR_CODE_WRONG_TXID              -8
+#define WALLET_RPC_ERROR_CODE_WRONG_SIGNATURE         -9